Ion implantation is a standard technique for introducing property-altering impurities into various substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at a front surface of the substrate. A common example is the change in electrical conductivity of a semiconducting material through the implantation of impurities such as boron, arsenic, etc. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
An ion implanter usually includes an ion source for converting a gas or a solid material into an ionized plasma from which a well-defined ion beam is extracted through the use of well established ion beam extraction electrodes. The ion beam may be mass analyzed to eliminate undesired ion species, accelerated to a desired energy and directed onto a target plane. The ion beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension. The long dimension may be at least as wide as the substrate.
Introducing the impurities at a desired density into the substrates is important to ensure that the semiconductor device being formed operates within specification. One factor that can affect the dose into the substrate is the ion beam current distribution within the ion beam. Accordingly, conventional ion beam current density measurement systems have been developed. One conventional ion beam current density measurement system translates a profiler sensor, e.g., a Faraday sensor as is known in the art, in a fixed direction through the ion beam. The measured beam current may be correlated to a known position of the traveling profiler sensor to provide an ion beam current distribution of the ion beam in one dimension. A drawback with this conventional approach is that it is limited to ion beam current density measurements in one dimension only, e.g., in the direction of travel of the traveling profiler sensor.
Another conventional ion beam current density measurement system is capable of measuring ion beam current density distribution in two dimensions by adding a plurality of pixels to the beam sensor. Each of the plurality of pixels operates as a beam current sensor and in one embodiment there may be two offset columns of pixels of about six pixels per column on a traveling profiler sensor. The traveling profiler has the advantage of using fewer pixels to get the beam density over a large area whereas a stationary set of pixels has a much reduced spatial resolution for the same number of pixels. However, a drawback with this approach is the cost and complexity of the additional pixels and associated wiring for each pixel. In addition, the pixels and associated wiring require space in an area of the ion implanter where there is a premium on such space. For example, it may be beneficial to translate the traveling profiler sensor outside the process chamber after measurements are taken. An associated opening in the process chamber may not be large enough to permit a large number of pixels and associated wiring to be coupled to the traveling profiler sensor.
Accordingly, there is a need in the art for a new and improved beam density measurement system for measuring beam density in two dimensions.